Method and apparatus for removing prosthetic cement

ABSTRACT

The removal of prosthetic cement from the medullary canal of a patient&#39;s long bone such as the femur during replacement of a joint prosthesis. The process comprises the steps of: (a) determining by X-ray or CT scan the profile of a line of intersection of a longitudinal plane, which extends longitudinally through the bone, the prosthetic cement and a prosthetic cavity therein, and a cement/bone interface; (b) determining the thickness of the prosthetic cement in the phase and along the line from the X-ray or CT scan; (c) forming a substantially planar cutting blade with a cutting edge having a profile substantially corresponding to the profile of the line; (d) forming on the cutting blade depth limiting members which are spaced from adjacent points on the cutting edge by a distance substantially equal to the depth of the prosthetic cement at the corresponding points along the plane; (e) inserting the cutting blade into the prosthetic cavity with the blade lying in the plane and to a depth such that corresponding points on the cutting edge are adjacent corresponding points on the line; (f) causing the cutting edge to reciprocate; (g) cutting into the prosthetic cement with the blade until the depth limiting members engage with the surface of the prosthetic cavity adjacent the plane; (h) repeating steps (a) to (g) to form at least one other cut through the prosthetic cement and (i) removing the segments of prosthetic cement so formed from the medullary canal of the bone. The process allows easier and faster removal of the prosthetic cement during joint prosthesis replacement with reduced possibility of bone damage.

TECHNICAL FIELD

The present invention relates to an improved process for the removal ofprosthetic cement from the medullary canal of a patient's long boneduring the replacement of a joint prosthesis, and to a cutting blade foruse therein.

BACKGROUND ART

The present inventor has proposed in PCT patent applicationPCT/AU86/00321 that the removal of prosthetic cement from the medullarycanal of a patient's long bone can be facilitated during a jointprosthesis replacement operation by making two or more longitudinallyextending cuts through the bone cement after removal of the oldprosthesis. These cuts divide the originally tubular bone cement into anumber of segments which may be conveniently levered away from theassociated bone and removed from the medullary canal. In theaforementioned patent application means were described for forming suchcuts through the prosthetic cement substantially without cutting intothe underlying bone. These means included guide means to guide a sawblade along the line of intersection of a longitudinal plane extendingthrough the bone, the prosthetic cement and a prosthetic cavity thereinand a bone/prosthetic cement interface intersected by the plane.

DISCLOSURE OF THE INVENTION

The present inventor has now discovered that these guide means may bedispensed with if a cutting blade with a suitably profiled cutting edgeis provided with depth limiting means which can bear against the surfaceof the prosthetic cavity adjacent the said plane to limit the depth towhich the blade can cut into the prosthetic cement along the plane tothe depth of the prosthetic cement along that plane.

The present invention thus consists in a process for the removal ofprosthetic cement from the medullary canal of a patient's long boneduring replacement of a joint prosthesis, comprising the steps of (a)determining the profile of a line of intersection of a plane, whichextends longitudinally through the bone, the prosthetic cement and aprosthetic cavity therein, and a cement/bone interface intersected bythat plane, (b) determining the thickness of the prosthetic cement inthe said plane and along the said line, (c) forming a substantiallyplanar cutting blade with a cutting edge having a profile substantiallycorresponding to the profile of the line, (d) forming on the cuttingblade depth limiting means, the depth limiting means being spaced fromadjacent points on the cutting edge by a distance substantially equal tothe depth of the prosthetic cement at the corresponding points along thesaid plane, (e) inserting the cutting blade into the prosthetic cavitywith the blade lying in the said plane and to a depth such thatcorresponding points on the cutting edge are adjacent correspondingpoints on the said line, (f) causing the cutting edge to reciprocate,(g) cutting into the prosthetic cement with the blade until the depthlimiting means engage with the surface of the prosthetic cavity adjacentthe said plane, (h) repeating steps (a) to (g) to form at least oneother cut through the prosthetic cement, and (i) removing the segmentsof prosthetic cement so formed from the medullary canal.

In another aspect the present invention consists in a cutting blade foruse in the removal of prosthetic cement from the medullary canal of apatient's long bone during the replacement of a joint prosthesis,comprising a substantially planar blade having a cutting edge, the edgehaving a profile substantially corresponding to the profile of the lineof intersection of a plane, which extends longitudinally through thelong bone, the prosthetic cement and a prosthetic cavity therein, and acement/bone interface intersected by that plane, and cutting depthlimiting means on the blade, the depth limiting means being spaced froman adjacent point on the cutting edge by a distance substantially equalto the depth of the prosthetic cement at the corresponding point alongthe said plane.

The blade is preferably formed with saw teeth along the cutting edge.The depth limiting means may comprise a rib extending along the lengthof the blade or, more preferably, a series of stops spaced apart alongthe length of the blade. In a particularly preferred embodiment of theinvention the depth limiting means comprises three pegs secured in holesin the blade and each projecting slightly on either side of the blade.It is preferable that at least two, more preferably at least three andmost preferably four cuts are made through the prosthetic cement. Thecuts are preferably equiangularly disposed around the prosthetic cavity.

The process and the cutting blade according to this invention areparticularly suitable for use in replacement hip prosthetic operationsin which an old prosthesis is removed from the femur and replaced with anew prosthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view through a femur into which anartificial hip prosthesis has been fixed;

FIG. 2 is a vertical sectional view through the femur of FIG. 1 with theprosthesis removed and showing by cross hatching an area of theprosthetic cement to be removed by sawing;

FIG. 3 is a vertical sectional view through the femur of FIG. 2 showingthe base of the prosthetic cement being drilled out;

FIG. 4 is a vertical sectional view through the femur of FIG. 3 with areciprocating saw blade in position after having made a saw cut throughthe prosthetic cement; and

FIG. 5 is a transverse sectional view along V--V of FIG. 4.

BEST MODE OF CARRYING OUT THE INVENTION

The invention is hereinafter described for the replacement of thefemural part of a total hip replacement prosthesis with reference to thedrawings hereinbefore identified.

The femur 10 of a patient has in the medullary canal 11 a hip prosthesis12 comprising a stem 13 and head 14. The prosthesis 12 was originallyfirmly held in place in the medullary canal 11 by prosthetic cement 15comprising polymethyl methacrylate. Over time the prosthesis 12 and thecement 15 have worked loose in the medullary canal 11 of the femur 10.

The hip prosthesis 12 can usually be easily removed from the cement 15by pulling the prosthesis 12 longitudinally of the bone. The withdrawnstem 13 leaves a cavity 16 in the cement 15. The problem then remains ofhow to remove the cement 15 without damaging the bone 10.

In the process according to this invention the patient is, prior toremoval of the old prosthesis, examined using X-rays or a CT scanner. IfX-ray examination is used one X-ray photograph is taken from the frontand one from the side of the patient to define the geometry of the stem13 relative to the bone 10 along two defined planes and thus theconfiguration of the cement 15 along these same planes. The CTexamination can also provide this information.

The thickness of the cement 15 is measured along the above planes byexamination of the X-ray photographs or CT scan. The thickness of onecut through cement 15 is shown by cross-hatching in FIG. 2. Appropriatejig saw blades 17 can then be made each having a cutting edge profile 18corresponding to the profile of the cement 15 bone 10 interface alongone of the defined planes and each having projections 19 spaced from thecutting edge 18 of the blade 17 by a distance corresponding to thethickness of the cement 15 adjacent the stop means 19 along that plane.

The saw blades 17 are preferably cut using a computer guided laser mill.If desired computer aided design (CAD) software may be used to transformthe X-ray images or the CT scan directly into instructions for the lasermill. The projection 19 may be formed by drilling holes at appropriatepositions on each blade 17 and inserting a small rod through each holesuch that each rod projects slightly on each side of the blade 17 andsecuring it in position in the blade as by welding.

Once the old prosthesis 12 has been withdrawn from the femur 10 a holeis drilled in the blind end 21 of the prosthetic cement 15 using aflexible drill bit 22 driven by an electric or pneumatic drill 23 andguided by a drill guide 24. The drill guide 24 comprises a tapered tube25 adapted to fit within the recess 16 in the cement 15 left bywithdrawal of the prosthesis and a handle 26. After the drill guide ispositioned within the recess 16 adjacent the blind end 21 the drill bitis inserted into the drill guide 24 and used to drill a hole through theblind end 21 of the cement.

The saw blade 17 is then attached to a pneumatically drivenreciprocating saw (partly shown at 27) and inserted into the recess 16.The saw blade 17 should be aligned in the correct plane and be insertedto the correct depth and then caused to cut into the cement 15. Theprojections 19 will abut against the inside surface of the cement 15when the saw blade 17 has cut through the full thickness of the cement15.

If the saw blade 17 is too wide to fit within the recess 16 apreliminary, shallow, cut may be made with a second saw blade (notshown) to allow the insertion of saw blade 17.

Once the cut with saw blade 17 is completed the procedure is repeated afurther three times to cut the cement 15 into four segments. Each ofthese four segments can then be readily levered away from the femur 10and withdrawn longitudinally from the medullary canal.

In practice it has been found that a slight cutting into the bone 10beneath the cement 15 is acceptable. This permits the use of apre-formed set of saw blades having shapes corresponding to internalshapes commonly found in femurs. A surgeon may select from such apreformed set the saw blade which has the best fit for each of theparticular cuts which is to be made on a particular patient. It is to beunderstood that the use of such a preformed saw blade is encompassedwithin the scope of the present invention.

I claim:
 1. A process for the removal of prosthetic cement from themedullary canal of a patient's long bone during replacement of a jointprosthesis, comprising the steps of (a) determining the profile of aline of intersection of a plane, which extends longitudinally throughthe bone, the prosthetic cement and a prosthetic cavity therein, and acement/bone interface intersected by that plane, (b) determining thethickness of the prosthetic cement in the said plane and along the saidline, (c) forming a substantially planar cutting blade with a cuttingedge having a profile substantially corresponding to the profile of theline, (d) forming on the cutting blade depth limiting means, the depthlimiting means being spaced from adjacent points on the cutting edge bya distance substantially equal to the depth of the prosthetic cement atthe corresponding points along the said plane, (e) inserting the cuttingblade into the prosthetic cavity with the blade lying in the said planeand to a depth such that corresponding points on the cutting edge areadjacent corresponding points on the said line, (f) causing the cuttingedge to reciprocate, (g) cutting into the prosthetic cement with theblade until the depth limiting means engage with the surface of theprosthetic cavity adjacent the said plane, (h) repeating steps (a) to(g) to form at least one other cut through the prosthetic cement, and(i) removing the segments of prosthetic cement so formed from themedullary canal.
 2. A process as claimed in claim 1 in which at leastthree cuts are formed in the prosthetic cement along three separateplanes.
 3. A process as claimed in claim 2 in which the cuts aresubstantially equiangularly spaced around the prosthetic cavity.
 4. Aprocess as claimed in claim 3 in which a preliminary cut partiallythrough the thickness of the prosthetic cement along the said plane ismade before step (e) to allow for the insertion into the prostheticcavity of a cutting blade wide enough to cut through the full thicknessof the prosthetic cement on that plane.
 5. A cutting blade for use inthe removal of prosthetic cement from the medullary canal of a patient'slong bone during the replacement of a joint prosthesis, comprising asubstantially planar blade having a cutting edge, the edge having aprofile substantially corresponding to the profile of the line ofintersection of a plane, which extends longitudinally through the longbone, the prosthetic cement and a prosthetic cavity therein, and acement/bone interface intersected by that plane, and cutting depthlimiting means on the blade, the depth limiting means being spaced froman adjacent point on the cutting edge by a distance substantially equalto the depth of the prosthetic cement at the corresponding point alongthe said line.
 6. A cutting blade as claimed in claim 5 in which thecutting blade is formed along its cutting edge with saw teeth.
 7. Acutting blade as claimed in claim 5 in which the depth limiting meanscomprises a rib extending along the length of the cutting blade.
 8. Acutting blade as claimed in claim 5 in which the depth limiting meanscomprises a series of stop members spaced apart along the length of thecutting blade.
 9. A cutting blade as claimed in claim 8 in which thedepth limiting means comprises three pegs secured in holes in thecutting blade and projecting slightly from either side of the blade.